VOIP networks must perform all of the same tasks that the PSTN does, in addition to performing a gateway function to the existing public network. There are three major pieces to IP networks.

-media gateways/signalling controllers
-media gateways
-voice over IP networks

Media gateways have optional features, such as voice (analog and/or digital) compression, echo cancellation, silence suppression, and statistics gathering.

Typically, each conversation (call) is a single IP session transported by a Real-time Transport Protocol (RTP) that runs over UDP.Their features and services can include some or all of the following.
-Trunking gateways that interface between the telephone network and a IP network.
-Business media gateways that provide a traditional digital PBX interface or an integrated soft PBX interface to a IP network.
-Access media gateways that provide a traditional analog or digital PBX interface to a IP network. Examples include small-scale (enterprise) IP gateways.
-Business media gateways that provide a traditional digital PBX interface or an integrated soft PBX interface to a IP network.
-Discreet IP telephones units.
-Network access servers that can attach a modem to a telephone circuit and provide data access to the Internet.

-Trunking gateways that interface between the telephone network and a IP network.
-Business media gateways that provide a traditional digital PBX interface or an integrated soft PBX interface to a IP network.
-Access media gateways that provide a traditional analog or digital PBX interface to a IP network. Examples include small-scale (enterprise) IP gateways.
-Business media gateways that provide a traditional digital PBX interface or an integrated soft PBX interface to a IP network.
-Discreet IP telephones units.
-Network access servers that can attach a modem to a telephone circuit and provide data access to the Internet.
Media Gateway Controllers
The media gateway controller has the responsibility for some or all of the call signaling coordination, phone number translations, host lookup, resource management, and signaling gateway services to the PSTN (SS7 gateway). Media gateway controllers house the signaling and control services that coordinate the media gateway functions. Media gateway controllers could be considered similar to that of H.323 gatekeepers.The amount of functionality is based on the particular IP enabling products used.

You could use a signaling gateway controller to directly connect to the SS7 network, while also interfacing to voice over IP networks elements. For calls that originate and terminate within the domain of the IP network, only a media gateway controller might be needed to complete calls. In a scalable voice over IP network, you can breakup the role of a controller into signaling gateway controller and media gateway controller.

Knowing the details of how the devices use their suite of protocols is important to designing the IP backbone that is to service the IP elements. The services of these devices are defined by the protocols and software they are running. There are several protocols and implementations that any number of vendors could deploy.

Depending on the IP protocols used, this system as a whole is sometimes referred to as a softswitch architecture.

Jim Francisto
business voip provider